General:
* Geoscience
* Earth science
* Geophysics
* Geochemistry
* Mineralogy
More specific:
* Petrology (study of rocks)
* Stratigraphy (study of rock layers)
* Paleontology (study of fossils)
* Structural geology (study of the Earth's structure)
* Geomorphology (study of landforms)
* Hydrogeology (study of groundwater)
The best word to use will depend on the specific aspect of geology you are referring to.